In color television and allied display applications, it is conventional practice to utilize cathode ray tubes of the type employing a patterned multi-phosphor cathodoluminescent screen interiorly disposed on the viewing panel of the tube envelope wherein an apertured or multi-opening mask member is spatially oriented relative thereto. A plurality of electron beams, emanating from an electron gun assembly, positioned within the neck portion of the envelope, are directed to converge at and traverse the apertured mask to impinge and luminescently excite the electron responsive phosphors comprising the patterned screen therebeyond. Focusing of the respective electron beams is conventionally achieved by means of discrete electron lensing; as for example, bi-potential focus lensing, such being dependent on the ratio of the focus voltage (G3) to the accelerating electrode or anode voltage (G4). With the advance of cathode ray tube technology, there has been a trend toward miniaturization and compaction of electron gun structures, which in turn, are encompassed within envelope neck portions of smaller diameters and shorter lengths. Consequently, the dimensionings and constructions of the electrode elements of the multi-gun assembly have been adapted to achieve the desired compaction. Such is especially evident in the conventional in-line plural gun assembly, wherein three separate electron beams emanate in a substantially common horizontal plane. This is accomplished by employing a unitized construction, in which several of the respective electrode elements, ahead of the individual cathodes, are unitized electrode members of definitive construction, each bearing at least one plane with three spatially-related in-line apertures therein.
While this unitized construction is beneficial in achieving the described structural results, it has been noted in a bi-potential gun assembly, that one or more of the beam landing spots at the center of the screen often tend to be of an ovate cross-sectional shaping, rather than the desired circular shape. This is particularly noticed relative to the beam projected by the center gun of the assembly, and appears to be the result of structural influences inherent in the unitized construction which introduce asymmetries into the beam lensing field effected within the main focusing electrode. The resultant focused ovate shaping of the beam landing area, at screen center, is deleterious to picture resolution in the screen display.